1. Field of the Invention
This invention relates to an image-forming optical system for forming the image of an object illuminated by an auxiliary illuminating system and, more particularly, to a high resolution image forming optical system improved in the image-forming performance.
2. Description of the Prior Art
As means of improving the resolution in an image forming optical system with an auxiliary illuminating system as a microscope, IC printer or outline projector, there have been used the following methods. One of them is a method wherein the numerical aperture (N.A.) is made large as carried out in such objective as, for example, of a microscope. However, as a result of improvements for a long time, the N.A. of an objective has become so large as to be substantially close to the limit and it is very difficult to design an objective of N.A. larger than that of today. Therefore, not only the image forming optical system but also the illuminating system is expected to be improved because generally the resolution of an optical system provided with such auxiliary optical system as is mentioned above depends on the coherence of the illuminating system and therefore the improvement of the illuminating method contributes to the elevation of the resolution. An annular illuminating system is known as one of such illuminating method in which the resolution is improved by utilizing this fact. It is a method wherein an annular aperture stop is arranged in the same position as an aperture stop usually placed in a microscope or the like and is made a secondary light source to illuminate an object. This annular illuminating method generally increases the high frequency response but reduces the low frequency response. Therefore, it reduces the contrast of the image. Thus, it has defects that no effect can be developed in observing the microstructure of specimen with low contrast such as a biotic object and a substantially high resolution image formation is hard to obtain.
The inventor of the present invention has already filed a Japanese patent application No. 88428/1979 (See the gazette of Japanese patent laid open No. 12615/1981) suggesting a high resolution optical system in which the above mentioned defects are eliminated.
It is an optical system of such formation as is shown in FIG. 1 in which an annular aperture stop 1 is arranged near the front side focal plane of a condenser lens 2 and is illuminated from the left side in the drawing by a light source (not illustrated) so as to make the aperture of this annular aperture stop a secondary light source to illuminate an object surface 3 through the condenser lens 2. An annular absorbing filter 5 is arranged in a position in which the image of the annular aperture stop 1 is formed by the light (0th diffracted) having directly entered an objective 4 without being diffracted by the object among the illuminating lights having thus illuminated the object so that the object image may be formed on an expected image plane 6 by the 0th diffracted light properly weakened by this annular absorbing filter 5 and the 1st, 2nd, . . . diffracted lights. It has been made clear that, according to the optical system of such formation, a high response is shown from a low frequency range to a high frequency range (spatial frequency) and a high resolution image formation is obtained.